1. Field of the Invention
The invention in general relates to atomic frequency standards, and more particularly to components of a physics package for an atomic clock of the type which utilizes an optically pumped cell containing a vapor.
2. Description of Related Art
Atomic clocks are utilized in various systems which require extremely accurate and stable frequencies, such as in bistatic radars, GPS (global positioning system) and other navigation and positioning systems, as well as in communications, cellular phone systems and scientific experiments, by way of example.
In one type of atomic clock, a cell containing an active medium such as cesium (or rubidium) vapor is irradiated with both optical and microwave energy whereby light from an optical source pumps the atoms of the vapor from a ground state to a higher state from which they fall to a state which is at a hyperfine wavelength above the ground state. The microwave signal is tuned to a particular frequency so as to repopulate the ground state. In this manner a controlled amount of the light is propagated through the cell and is detected by means of a photodetector.
By examining the output of the photodetector, a control means provides various control signals to ensure that the wavelength of the propagated light and microwave frequency are precisely controlled.
There is a need, both in the military and civilian sectors, for an ultra small, completely portable, highly accurate and extremely low power atomic clock. The atomic clock must operate continuously for 24 hours per day to perform a useful function. For this reason, power levels approaching 100 milliwatts, or less, are desirable for military and many civilian uses.
The non-electronic portion of the atomic clock is often referred to as the physics package and, as will be described, includes power consuming elements such that the physics package promises to be the determiner of the size, low power capabilities and ultimate low cost of the final product.
It is a primary object of the present invention to provide physics package apparatus for an atomic clock, which is of small size, for example, 1 cm3, or less, and which meets low cost, ease of fabrication and low power usage requirements.
Physics package apparatus for a cell type atomic clock in accordance with the present invention includes a cell structure having a central plate sandwiched between top and bottom plates. The central plate has a central interior aperture which together with the top and bottom plates forms an internal cavity for containment of an active vapor. The central plate includes a reservoir for holding a source of the active vapor, and a channel connecting the reservoir with the internal cavity.
Apertures on either end of the central plate respectively accommodate a laser diode, which projects a laser beam through the vapor, and a detector for detecting the projected beam. End walls of the interior aperture may include curved portions for shaping and focusing the laser beam.
A microwave coupling arrangement, such as strip line conductors couples microwave energy into the vapor-containing cavity. For a complete physics package, insulation, a C-field coil and a surrounding metallic magnetic shield may be included.